This invention relates to Coriolis flowmeter and, in particular, to Coriolis flowmeters having a single flow tube connected to a surrounding balance bar. This invention further relates to a straight flow tube Coriolis flowmeter having a balance bar whose design and construction reduce the residual thermal stress on the flow tube due to manufacturing operations involving temperatures exceeding those encountered during normal operating conditions of the flowmeter.
Coriolis flowmeters having a single straight flow tube surrounded in part by a balance bar are known. The balance bar counterbalances the vibrating flow tube so that the balance bar and flow tube together form a dynamically balanced structure. The outer axial ends of the balance bar are connected by connecting rings to the outer surface of the flow tube. These connections are accomplished by a high temperature bonding operation such as by brazing or soldering.
The thermal stresses and corrosive fluids to which flow tubes are subjected in normal operation require that they be formed of titanium. A titanium flow tube is readily brazed to a titanium balance bar and, since both have the same thermal expansion coefficient, the cooled assembly has very little residual thermal stress. Unfortunately, titanium balance bars are very costly. Balance bars could be made of less expensive steel except for the problem of high temperature brazing. Steel has a thermal expansion coefficient that is close enough to titanium so that under normal flowmeter operating temperatures neither the flow tube nor the balance bar are over-stressed due to differential thermal expansion. However, the high temperature brazing operation used to join the parts (more than 746xc2x0 Centigrade) expands the steel balance bar significantly more than the titanium flow tube. As these parts begin to cool, the braze material solidifies at a temperature well above the normal operating temperature range of the flowmeter. Continued cooling shrinks the steel balance bar axially more than the titanium flow tube. This differential shrinkage can amount to a strain level of more than 2.3 mm per meter of balance bar length. A shrinkage of this magnitude puts the titanium flow tube in compression with a stress of 0.23 GPa and can impair the performance of the flowmeter or can even exceed the yield strength of the flow tube under certain conditions.
Attempts have been made to overcome the differential thermal expansion/contraction problems associated with the use of a titanium flow tube with a balance bar formed of a different material such as steel. However, these attempts have not been wholly successful and have resulted in the creation of other problems. One such attempt involves the use of a steel balance bar having two half sections joined by bellows as a center portion. This solution is not ideal since bellows are axially symmetric and make difficult the controlling of the vibrational modes of the balance bar. Another solution, is the joining of separate balance bar sections by means of leaf springs. This solution also results in problems in the vibrational characteristics of the balance bar.
It can therefore be seen in view of the above that it is a problem to provide a Coriolis flowmeter having a titanium flow tube and a steel balance bar that are not susceptible to structural damage during the manufacturing operations involving the use of high temperature bonding operations.
The residual thermal stress problem is solved by separating the balance bar structure into two independent halves. These halves are separated from each other in the central portion so that, in the braze furnace, the balance bar halves can expand and contract without contacting each other and without stressing the flow tube. At this time, the balance bar halves are brazed to the flow tube via connecting rings at their axially outer ends.
A significant aspect of this invention involves the method in which the balance bar halves are connected to each other after the brazing step. This connection is necessary for several reasons. First, it is necessary for the tuning of the modes of vibration of the balance bar-flow tube assembly. This assembly is a distributed mass-stiffness system and therefore has an infinite number of modes of vibration. One of the important modes of vibration is the drive mode in which the flow tube and the balance bar vibrate out-of-phase with each other in the drive plane direction in their first bending modes. There is another vibration mode which looks like the drive mode except that it occurs in a direction perpendicular to the drive mode. This mode is called the lateral mode. If the lateral mode frequency is too near the drive mode frequency, the accuracy of the meter is reduced. The prior art bellows connection means has equal bending stiffnesses in both the drive and lateral directions resulting in nearly equal resonant frequencies. The present invention separates these two frequencies by making the connection means between the two balance bar halves stiffer in bending in the drive direction than in the lateral direction. This raises the drive mode frequency above the lateral mode frequency.
Connecting the two balance bar halves also enables both halves to be driven in the drive mode by a single driver. This is important because two drivers would have to be precisely matched in order to avoid deforming the flow tube in a shape that looks like a response to fluid flow. Such deformation would give erroneous flow readings.
The present invention overcomes the above discussed and other problems resulting from high temperature induced stress on the flow tube during the brazing of the balance bar to the outer walls of the flow tube via connecting rings. This new process includes the steps of providing a balance bar having separate halves; bonding the outer axial ends of the balance bar halves via connecting rings to the outer walls of the flow tube; connecting balance bar halves to each other by side channel members which are positioned parallel to the longitudinal axis of the balance bar halves and are bisected by the neutral surface of the assembly in the drive mode. (The neutral surface in the drive mode is defined as the theoretical surface in a bending member that experiences neither compressive nor tensile stresses. In the single tube flowmeter it is defined by the tube axis and a line intersecting it that extends in the lateral direction.) Furthermore, the side channels are radially spaced apart from the outer surface of the balance bar. The side channels are connected to the balance bar by means of pegs. The pegs are inserted into holes in the channel members and holes in the balance bar halves.
The pair of cylindrical balance bar halves has a combined axial length somewhat less than that of a conventional integral balance bar. The inner axial ends of the balance bar halves are separated a desired amount to form a center section that separates the balance bar halves. The flow tube is inserted within the interior of the balance bar halves. The outer axial ends of the balance bar halves are aligned with a connecting ring having a center opening through which the flow tube is extended. The outer periphery of the connecting ring is then brazed to the axial outer end of each balance bar half and the inner surface of the connecting ring is simultaneously brazed to the outer surface of the flow tube.
During the same brazing operation, the pegs are brazed into holes in the walls of each balance bar half. The hole centers are located on the neutral surface. At this time, the fabrication of the flowmeter has proceeded to the point where the outer ends of the balance bar halves are brazed to the flow tube via the connecting rings and the pegs are brazed into the holes in the balance bar walls.
Following the brazing operation the pegs are inserted into holes in the side channels and welded. Each balance bar half is connected by a welding operation to each side channel using a pair of pegs for each side channel. A greater number of pegs such as, for example, four pegs per channel (two on each end) could also be used. At this point in the fabrication process, the assembly includes a flow tube inserted within the interior of a pair of axially aligned balance bar halves. Connecting rings bond the outer axial ends of each balance bar half to the flow tube walls. Pegs are brazed to holes in the outer walls of the balance bar halves and protrude outward from the balance bar walls radially. Two side channels are positioned onto the pegs on the opposite sides of the balance bar halves and are welded to the pegs.
The pegs are used as intermediate parts between the balance bar halves and the side channels for three reasons. First, welding the channels directly to the balance bar halves would produce welds that would be long and parallel to the meter axis. This would produce a great amount of heat, shrinkage, and ultimately residual tube stress. Second, the stress in these welds due to the drive vibration would be greatest at the ends of the welds adjacent to the gap between the balance bar ends. The ends of welds also are the sites of the highest welding stress. Superimposing the cyclic vibration stresses of normal operation onto the high weld stress at the weld ends would almost certainly result in fatigue cracking. Finally, it is difficult to make full penetration welds in the long fillet welds between the side rails and the balance bar halves. It is important that the welds between the balance bar halves and the side channels achieve full penetration into the side channels so there is no possibility of a rubbing friction between the balance bar halves and the channels. Any such rubbing friction impairs the stability of the output data of the resultant Coriolis flowmeter.
Pegs avoid these problems. The welds are small and localized and thus put very little heat into the side rails or the balance bar halves. This results in less axial shrinkage of the side rails. The side channels are radially spaced away from the balance bar halves so that the only contacts with the balance bar halves are via the pegs. The end of the peg is easily given a chamfer to allow a full penetration weld and eliminate rubbing friction in these welds. Also, the weld of the peg to the side channel is circular and has no high weld stress location like the linear weld. Finally, the cyclic vibration stress applied to the peg is primarily torsion which is uniformly distributed throughout the weld.
The balance bar halves and the side channels and pegs comprise a rigid, axially elongated member which vibrationally functions in the same manner as an integral balance bar. The balance bar halves connected by the side channels can be vibrated transversely in phase opposition to the flow tube to form a dynamically balanced member having performance characteristics comparable to that of an integral balance bar. Furthermore, the use of the balance bar halves together with the side channels and the pegs eliminate the destructive forces associated with the high temperature brazing of a flow tube made out of a first material, such as titanium, to a balance bar made out of a different material, such as steel.
The affixing of the side channels to the balance bar via pegs and a welding operation is an improvement over the prior art flowmeters which generate undesirable axial stress in the flow tube and/or the balance bar as a result of the single high temperature brazing operation. Welding produces its own shrinkage that slightly shortens the channels and produces tube stress. However, welding heating is fast and localized and thus results in shrinkage and residual tube stress that is factor of ten less than that resulting from furnace brazing. Furthermore, other aspects of the design of the present invention reduce residual tube stress due to the weld shrinkage to an insignificant level.
The present invention permits the cost of a single tube flowmeter to be reduced by permitting a steel balance bar or the like to be used in combination with a titanium flow tube. The advantages of the present invention are achieved by the use of balance bar halves rather than a single integrated balance bar. These advantages are still further achieved by the means of the use of pegs which are brazed to the balance bar and are subsequently welded to the side channels. The side channels are made from readily available stock material such as rolled mild steel channel. The pegs are made from readily available steel tubing.
An aspect of the invention is a Coriolis flow meter adapted to be connected to a system having a material flow, said Coriolis flow meter having a balance bar and a flow tube adapted to be vibrated in a drive plane in phase opposition to generate Coriolis deflections of said vibrating flow tube with material flow; pickoff means that detect said Coriolis deflections to generate signals representing information pertaining to said material flow; said balance bar is coaxial with said flow tube and surrounds a portion of said flow tube, said Coriolis flow meter further comprises:
connecting rings;
separate halves of said balance bar each being coaxial with said flow tube and with each half having an axial outer end connected to a surface of said flow tube by one of said connecting rings;
elongated apparatus that interconnects the inner axial ends of said balance bar halves, said elongated apparatus is located on the neutral plane of the drive mode of said balance bar and is spaced apart from the outer radial surface of said balance bar halves;
said elongated apparatus and said balance bar halves vibrate in said drive plane as an integral structure.
Preferably said elongated apparatus comprises:
a plurality of elongated elements each spaced apart from the outer radial surface of said balance bar halves and oriented parallel to the longitudinal axis of said balance bar halves.
Preferably said elongated elements define side channels that are-u-shaped in a cross section perpendicular to their longitudinal axes.
Preferably said elongated elements define side channels that are non-U-shaped in cross section.
Preferably pegs connect said elongated elements to said balance bar halves;
said pegs and said elongated elements and said balance bar halves vibrate in said drive mode as an integral element.
Preferably said pegs are located in holes in said elongated elements and in said balance bar halves.
Preferably said pegs are bonded at a first end to holes in said balance bar halves and welded at a second end to holes in said elongated elements.
Preferably said pegs are tubular with a hollow center core.
Preferably said pegs are circular in cross section.
Preferably said pegs have an non circular cross section.
Preferably at least one peg connecting each elongated element with each of said balance bar halves.
Preferably at least two pegs connecting each elongated element with each of said balance bar halves.
Preferably said pegs are tubular with each peg each having a first end bonded to one of said balance bar halves and a second end welded to one of said elongated elements.
Preferably said elongated elements have slots oriented transversely to a longitudinal axis of said elongated elements, said slots limit axial shrink of said elongated elements due to the welding of said pegs to said elongated elements.
Preferably said Coriolis flow meter has two elongated elements oriented longitudinally on opposite sides of said balance bar.
Preferably said weld defines an endless closed loop.
Preferably said weld defines a circular pattern.
Another aspect of the invention is a method of forming a Coriolis flow meter adapted to be connected to a system having a material flow, said Coriolis flow meter having a balance bar and a flow tube adapted to be vibrated in a drive plane in phase opposition to generate Coriolis deflections of said vibrating flow tube with material flow; pickoff means that detect said Coriolis deflections to generate signals representing information pertaining to said material flow; said balance bar is coaxial with said flow tube and surrounds a portion of said flow tube, said method comprises the steps of:
forming said balance bar to define separate cylindrical balance bar halves;
inserting said flow tube into the axial center of said balance bar halves so that at least a portion of said flow tube is surrounded by said balance bar halves;
bonding an axial outer end of each balance bar half to an outer surface of said flow tube using a connecting ring having a center opening through which said flow tube extends;
positioning elongated apparatus on the neutral plane of the drive mode of said balance bar halves and spaced apart from the outer radial surface of said balance bar halves; and
connecting said elongated apparatus to said balance bar halves so that said elongated apparatus and said balance bar halves vibrate as an integral unit when said flow tube and said balance bar halves are vibrated in said drive mode.
Preferably said elongated apparatus defines a plurality of elongated elongated elements and said method includes the further steps of:
positioning said elongated elements on opposite sides of said balance bar halves; and
connecting said elongated elements to said balance bar halves via said pegs.
Preferably said step of connecting said elongated elements to said balance bar halves includes the steps of:
forming holes in said outer surface of said balance bar halves;
forming matching holes in said elongated elements;
brazing a first end of tubular pegs to said holes in said balance bar halves;
inserting a second end of said pegs into said matching holes of said elongated elements; and
welding said second end of said pegs to said elongated elements.
Preferably said step of welding includes the step of forming said weld to define a circular pattern.
Preferably forming tubular pegs with each peg each having a first end bonded to one of said balance bar halves and a second end bonded to one of said elongated elements.
Preferably forming the walls of said elongated elements with slots oriented transversely to a longitudinal axis of said elongated elements to limit axial shrink of said elongated elements due to said step of welding.
Preferably forming said elongated elements to define elongated side channels that are-U-shaped in a cross section perpendicular to their longitudinal axes.
Preferably forming said elongated elements to define elongated side channels that are non-U-shaped.
Preferably connecting at least two pegs connecting each elongated element with each of said balance bar halves.
Preferably forming said Coriolis flow meter so that said pegs are circular in cross section.
Preferably forming said Coriolis flow meter so that said pegs have a non circular cross section.
Preferably forming said Coriolis flow meter so that there is at least one peg connecting each elongated element with each of said balance bar halves.